The present invention relates to breathing systems such as anesthesia systems, ventilating elements for use in such systems, and methods of administering ventilation.
A typical anesthesia system includes a breathing circuit for delivering a gas mixture including air or another oxygen-containing gas and an inhalation anesthetic agent to the patient. The breathing circuit typically includes one or more conduits connected to a mask or tube which communicates with the patient's lungs. The breathing circuit also may include elements such as devices for adding anesthetic agent to the gas mixture, adsorbers for removing undesirable constituents from the gas mixture, and the like. The conduits commonly are arranged in a closed loop, so that gas mixture continually circulates in the loop, with some or all of the exhaled gas passing back to the patient, in admixture with fresh gasses. The circuit is commonly provided with check valves which allow flow in one direction along the circuit but block flow in the opposite direction.
In many cases, the patient cannot breathe spontaneously. Therefore, the anesthesia system typically also includes an element referred to herein as a “ventilating element” defining a chamber of variable volume in communication with a conduit of the breathing circuit. The ventilating element can be repeatedly expanded to draw in gas and then forcibly collapsed to expel the gas into the conduit. When the ventilating element is collapsed, the check valves in the circuit assure that the gas expelled from the ventilating element is directed to the patient. This provides a positive pressure which overcomes the natural elastic recoil of the lungs and chest wall, and overcomes the resistance of patient's airways, and thus forces the gas into the lungs to provide inhalation. This procedure is commonly referred to as “positive pressure ventilation.”
A mechanical system can be used to provide positive pressure ventilation by automatically actuating the ventilating element. However, in many cases it is desirable to actuate the ventilating element manually. A skilled operator such as an anesthesiologist, anesthetist or other medical professional can provide the correct volume of the gas mixture for inhalation by feel, without the cost, complexity and reliability issues associated with an automatic ventilation system. Merely by way of example, manual ventilation is particularly advantageous where anesthesia must be administered in a setting where resources are limited as, for example, in remote rural regions of the world or in areas affected by natural disasters.
Ventilating elements which can be actuated manually include bags and bellows. A bag may be a soft, balloon-like structure which is normally slack, so that the bag is inflated by the gas pressure in the breathing circuit. Alternatively, a bag may have resilient walls which tend to return the bag to an inflated condition and thus draw gas into the bag. The bag may have one open end which is connected to the breathing circuit, or may have separate gas intake and gas outlet openings at opposite ends. The operator squeezes the bag to collapse it and provide positive pressure for inhalation. A bellows typically has a closed end, an open end and a series of pleats between the ends. The open end is connected to a conduit of the breathing circuit for entry and exit of gas. The operator can manually expand and collapse the bellows by moving the closed end towards and away from the open end.
The volume of gas delivered to the patient during each respiratory cycle, commonly referred to as the “tidal volume” must be selected to match the lung capacity of the patient. Children require considerably smaller tidal volumes than adults. If a ventilating element intended for use with adults is used to deliver positive pressure ventilation to a small child, the operator may accidentally deliver an excess tidal volume or excess positive inflating pressure to the child. When using a system equipped with a bag, the operator normally must select a bag having the correct size for the patient. This can lead to accidents if the wrong bag is installed.
Systems using bellows typically employ only one size of bellows. These bellows are sized to provide the correct tidal volume for an adult when operated through a reasonable stroke. To provide the correct tidal volume for a small child, the operator must use a very short stroke. In these conditions, it is difficult for the operator to estimate and control the volume administered. This increases the difficulty and risk of the procedure.
Kuhn, U.S. Pat. No. 5,520,173 (“the '173 patent”) discloses a tubular ventilating element having two open ends and an axis extending between these ends. The element can be connected between conduits of a breathing circuit. The element has sections of different diameters. The element is resilient, and returns to a fully-expanded condition under the influence of its own resilience. The operator can pump gas through the element by squeezing the element in directions transverse to the axis. According to the '173 patent, the operator can pump a large volume of gas by squeezing a large-diameter section of the element, or pump a small volume of gas by squeezing a small-diameter section of the element. Such an element requires an unnatural control action by the operator, is fatiguing for the operator, and suffers from other drawbacks.
Thus, despite considerable effort in the art, further improvement would be desirable.